Systematics of fission-channel probabilities

Fissioning nuclei produce with increasing excitation varying yields of frag ments. For compound nuclei ranging from Th-232 to Pu-242 endowed with excit ation energies typically between 0 and 10 MeV, we analyze these variations in terms of a fission-channel model. Most of the variations can be attribut ed to changing channel probabilities. We present a systematics of channel p robabilities with respect to compound nuclei and their excitation energies, and we relate the systematics to potential energy which the nuclei experie nce when they float to scission. The trend, which is most difficult to expl ain, is a shift in the energy sensitivity of the standard channels, as one compares light with heavy compound nuclei. According to their behavior, we divide nuclei into standard I increasers and decreasers and attribute the d ifference to the standard secondary barriers. As a basic concept, bifurcati on ratios are introduced, and a novel expression for transmission coefficie nts is proposed. [S0556-2813(99)05402-3].